Fix grpc-web trailer rendering for fast path

[jrudolph]

Turns out we lost trailer rendering in the performance optimization for strict entities. This is an attempt at adding it back. Would still be good to add some tests. I currently don't have time to develop this further but maybe we can prepare the fix for the next release.

Refs #1465

[wb14123]

Any plan to merge this in? This support is really important in my project. Otherwise my client will be broken because of the missing trailer.

[raboof]

@jrudolph I added the necessary MiMa filter, is there anything more you were planning to look into here because you marked it as 'Draft'?

[jrudolph]

I reworked the code.

I tested against akka-grpc-quickstart-scala and looked at the output which now looks reasonable when queried like this:

$ cur  'http://localhost:8080/GreeterService/SayHello' -X POST -H 'User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:96.0) Gecko/20100101 Firefox/96.0' -H 'Accept: application/grpc-web-text' -H 'Accept-Language: en-US,en;q=0.5' -H 'Accept-Encoding: gzip, deflate' -H 'custom-header-1: value1' -H 'Content-Type: application/grpc-web-text' -H 'X-User-Agent: grpc-web-javascript/0.1' -H 'X-Grpc-Web: 1' -H 'Origin: http://localhost:8081' -H 'Connection: keep-alive' -H 'Referer: http://localhost:8081/' -H 'Pragma: no-cache' -H 'Cache-Control: no-cache' --data-raw 'AAAAAAYKBGFvZXU=' | base64 -d |hexdump -C

00000000  00 00 00 00 0d 0a 0b 48  65 6c 6c 6f 2c 20 61 6f  |.......Hello, ao|
00000010  65 75 80 00 00 00 0f 67  72 70 63 2d 73 74 61 74  |eu.....grpc-stat|
00000020  75 73 3a 30 0d 0a                                 |us:0..|
00000026

[jrudolph]

While with 2.1.3 the output is this, i.e. missing the trailer completely:

00000000  00 00 00 00 0d 0a 0b 48  65 6c 6c 6f 2c 20 61 6f  |.......Hello, ao|
00000010  65 75                                             |eu|
00000012

[jrudolph]

More code is now reused between strict and streamed responses.

[jrudolph]

This line might be another bug I introduced in the optimization: we potentially report compression but don't actually do compression.

[jrudolph]

Would be nice if you could have another look at the implementation, @raboof, given that we have no tests.

[raboof]

Looks good, haven't tested it myself yet

[viktorklang]

@jrudolph I think Trailer could get some optimizations as well—having a cached instance for an empty Trailer, and avoiding creating a collection of 1 element and then immediately creating a new collection via map: https://github.com/akka/akka-http/blob/main/akka-http-core/src/main/scala/akka/http/scaladsl/model/Trailer.scala#L33

[jrudolph]

For akka-grpc we are already using a cached instance here: https://github.com/jrudolph/akka-grpc/blob/efda1b1ff4a8fd9849be66a5cd8c66369f6027af/runtime/src/main/scala/akka/grpc/internal/GrpcResponseHelpers.scala#L32. For akka-http itself, you could also just omit the trailer when not needed.